The combustion process which takes place within the combustor of a gas turbine engine results in the combustor walls being exposed to extremely high temperatures. The alloys which are typically used in combustor wall construction are normally unable to withstand these temperatures without some form of cooling arrangement. It is therefore known to make use of pressurised air derived from the engine's compressor for cooling purposes within the combustor.
One way of cooling the combustor wall with compressor air in this manner involves the provision of a double wall combustor construction having a continuous outer wall and an inner wall made up of a number of separate and replaceable wall elements in the form of tiles which are affixed to the outer wall in a tessellated manner. The inner wall tiles are each configured to be affixed to the outer wall of the combustor so as to define a chamber between a cooling side surface of the tile and the outer wall. The outer wall is provided with a number of feed holes through which cooling air drawn from the engine's compressor is directed so as to pass into the chambers defined between each inner tile and the outer wall, for impingement on the aforementioned cooling side surface of the inner tile, thereby providing impingement cooling to the inner tile. The inner tiles are each furthermore provided with a plurality of so-called effusion holes which define flow passages through the tiles from their cooling side surfaces to oppositely directed combustion side surfaces which face the interior of the combustor where combustion will take place during operation of the engine. The cooling air which is directed into the chambers and which impinges on the cooling side surface of the tiles is thus exhausted through the effusion holes and in doing so provides convective heat removal from the tiles. The air subsequently forms a thin film of air over the tiles' combustion side surfaces which helps to protect the tiles from the combustion flame inside the combustor. In order to aid the formation of this thin film of air, the effusion holes are often inclined relative to the combustion side surface. Combustor wall arrangements of the type described above thus provide both impingement and effusion cooling of the combustor wall construction, and the tiles are sometimes referred to as impingement/effusion (“IE”) tiles.
U.S. Pat. No. 5,435,139 describes a tile system of the general type described above. This document also shows how the tiles are typically affixed to the outer wall of the combustor. Each tile has a number of integrally-formed threaded studs which protrude outwardly from the cold side of the tile and which are received through respective apertures formed in the outer wall of the combustor and engaged by respective self-locking nuts on the outer side of the outer wall.
Tiles of the type described above are typically formed from a nickel based alloy, and have their combustion side surfaces protected by a thermal barrier coating to insulate the tile and thereby maintain the temperature of the metal within acceptable levels.
The thermal barrier coating is usually applied in two parts: an initial bond coat (such as a CoNiCrAly composition); and a thermally insulating top coat which may comprise Yttria Partially Stabilised Zirconia (“PYSZ”) and which is applied over the bond coat. The bond coat is applied directly to the metal of the tiles, for example by air plasma spray, to ensure adherence of the subsequent top coat. The bond coat may typically have a thickness of between 0.05 mm and 0.2 mm, whilst the top coat usually has a thickness of between 0.1 mm and 0.5 mm.
As will be appreciated, it is important for proper functioning of the tiles that their effusion holes are not blocked by the application of the thermal barrier coating. This represents a significant technical challenge, and various processes have been proposed in the prior art to prevent effusion hole blockage.
One such process, known as a so-called “coat-drill” process involves applying the thermal barrier coat to the combustion side surface of a tile, and then subsequently forming the effusion holes through both the alloy of the tile and the coating. This usually involves forming the holes either by mechanical drilling or by laser from the combustion side, firstly through the thermal barrier coating and then through the metal of the tile. Although this process is relatively simple, in the case of laser-cutting the effusion holes the laser must be operated at reduced power to avoid excessive damage to the brittle ceramic thermal barrier coating. Reducing the power of the cutting laser increases the cycle time necessary to form the holes which can significantly increase the production cost of the tiles. Furthermore, forming the effusion holes through the thermal barrier coating can cause cracking and delamination in the coating which can lead to premature loss of the coating during service, resulting in potential thermal damage to the tiles.
Alternatively, it is possible to form the effusion holes through the tile before the thermal barrier coating is then applied. This process, known as a so-called “drill-coat” process, is also relatively simple and has the benefit of allowing full-power operation of a cutting laser to form the effusion holes. However an inevitable consequence of this process is that some or all of the effusion holes then become either partially or completely blocked by the thermal barrier coating when it is applied. These blockages reduce the effective flow area of the tile and thus have a deleterious effect on convective heat removal within the effusion holes and the formation of a cooling film of air across the combustion side surface of the tile during service.
It is therefore considered preferable to use a so-called “drill-coat-clean” process, which is basically similar to the “drill-coat” process but which includes a subsequent cleaning process effective to clean the effusion holes to remove any coating material blocking the effusion holes. This cleaning step can be done via the use of a high pressure water or air jet, which may contain abrasive particles, and which is directed towards and through the holes to blast out any coating material therefrom. The water or air jet is usually directed towards the effusion holes from the cooling side of the tile. U.S. Pat. No. 8,262,802 discloses this type of technique.
A cleaning step of the type described above, carried out either after the entire thickness of the thermal barrier coating has been applied or as an intermediate step carried out after the initial bonding layer has been applied, has been found to provide clean effusion holes with slightly rounded edges. Also, the thermal barrier coating remains free from cracks and delamination which can arise via use of a laser to cut the holes after application of the coating.
However, in the specific context of a combustor liner tile, it can be difficult to direct the cleaning jet properly at all of the effusion holes because of obstruction by the attachment studs which project outwardly from the cold side of the tile. This problem is illustrated schematically in FIG. 1 which shows an IE tile 1 having a cooling side 2 and a combustion side 3. The cooling side 2 of the tile defines a cooling side surface 4, and the combustion side 3 of the tile defines a combustion side surface 5 which in use will be directed to the region of a combustor in which combustion will take place. The effusion holes 6 can be seen to extend between the cooling side surface 4 and the combustion side surface 5 at an inclined angle to the combustion side surface 5. FIG. 1 also illustrates a pair of externally threaded attachment studs 7 of the type described above in the prior art, which protrude from the cooling side 2 of the tile for receipt through respective apertures formed in the outer wall of a combustor (not shown). As will be appreciated, the attachment studs must have sufficient length to extend across the cavity which will be formed between the cooling side surface 4 of the tile and the outer wall of the combustor, and then project through the apertures in the outer wall by a sufficient degree to engage a threaded nut. A typical IE tile may have up to eight attachment studs 7 of this type, provided in spaced-apart relation to one another over the cooling side of the tile.
FIG. 1 also shows a cleaning nozzle 8 which is used to direct a jet of cleaning water or air towards the effusion holes 6 as illustrated, in order to clean the effusion holes of any coating material that may collect therein during the step of applying a thermal barrier coating to the combustion side surface 5 as described above. The nozzle 8 is positioned to direct a jet along a jet axis 9 towards each effusion hole 6, the jet axis 9 being inclined relative to the combustion side surface 5 by the same angle as the effusion holes so that the jet is directed through the holes. The nozzle 8 may be moved across the cooling side of the tile 1, for example in a scanning manner, to direct its cleaning jet though successive effusion holes.
However, it has been found that the length of the attachment studs 7, which can typically be approximately 15 mm, obstructs the nozzle 8 and can therefore prevent effective cleaning of the effusion holes 6. In order to clean the effusion holes effectively it has been found that the nozzle 8 should be spaced from the cooling side surface 4 by a distance of approximately 30 mm or less, as measured along the jet axis 9. The length of the attachment studs 7 precludes this because clashes occur between the nozzle 8 and the studs 7 as the nozzle is moved across the cooling side 2 of the tile at a range of anything less than 50 mm measured along the jet axis 9. Also the length of the studs 7 can also preclude the jet being properly directed towards several effusion holes proximate to each stud, those holes thus effectively sitting in the “shadow” of the studs.
Another problem which arises from the prior art configuration of the attachment studs 7 is that they represent a limiting factor in the efficiency with which the IE tiles can be manufactured by a Direct Laser Deposition (“DLD”) technique. DLD is a type of additive layer manufacturing technique which is considered to be advantageous for the production of IE tiles from their base alloy because it allows all features of the tiles, including the effusion holes and the attachment studs, to be formed integrally in a single process. In order to maximise the number of tiles which can be produced simultaneously via a DLD process it is optimal to form the tiles in a vertically stacked array on the DLD machine bed. However, it has been found that this orientation often produces an unacceptable quality of threads on the attachment studs of the tiles. Improved threads can be obtained by forming the tiles in a horizontally arranged array, but in this orientation the number of tiles which can be formed simultaneously in any given DLD machine is significantly reduced, which thus increases the production cost per tile.